feat: add ESLint configuration and testing infrastructure

Add comprehensive ESLint setup with React and TypeScript support, create basic integration tests for the shutdown utilities, and enhance the Justfile with a new check command that runs all validation steps (cargo check, clippy, tests, and linting).

web/src/lib/api.ts

@@ -43,7 +43,7 @@ export class BannerApiClient {
       );
     }
 
-    return response.json();
+    return (await response.json()) as T;
   }
 
   async getHealth(): Promise<HealthResponse> {